以樹脂可抽出 Cr(III) 及 Cr(VI) 評估鉻污染土壤中之鉻植物毒性

Abstract

由於土壤中之 Cr(VI) 具強氧化性及高移動性，因此在評估鉻植物毒性時通常著重在Cr(VI)，然酸性土壤中之 Cr(III) 植物毒性可能會因為其溶解度的增加而提高，因此 Cr(III) 之植物毒性不容忽視。另外，酸性土壤中之 Cr(VI) 易被存在土壤中的電子供應者如有機質還原成Cr(III)，而 Cr(III) 之溶解度會隨著土壤 pH 值的下降而增加而導致其植物毒性的加劇，一旦酸性土壤遭受 Cr(VI) 污染，Cr(VI) 和 Cr(III) 皆可能導致植物毒害，故 Cr(VI) 污染土壤中不同鉻形態之植物毒性評估亦有其必要性。本研究擬以Chelex 100 樹脂評估鉻污染土壤中 Cr(III) 之有效性，並以小麥幼苗試驗對樹脂抽出結果進行驗證。另以 Chelex 100 及 Dowex M4195 兩種選擇性離子交換樹脂來評估添加 Cr(VI) 酸性土壤中 Cr(III) 及Cr(VI) 之有效性及土壤 pH 值對兩種鉻形態有效性之影響，並以X-射線近邊緣結構圖譜 (XANES) 鑑定添加 Cr(VI) 酸性土壤中 Cr(VI) 之還原程度以驗證樹脂抽出結果，並以小麥生長試驗觀察鉻之植物毒性。 在酸性土壤中，Chelex 100 樹脂之鉻抽出量會隨著Cr(III) 添加濃度之增加而增加，但在鹼性土壤中其鉻抽出量則低於儀器之偵測極限。酸性土壤中小麥之株高及乾重與樹脂抽出結果呈反比，且植物受害情形越嚴重則植體中之鉻濃度越高，而鹼性土壤中之小麥株高及乾重並未因為Cr(III) 之添加而受害。因此證實Chelex 100樹脂抽出方法可有效用來評估土壤中Cr(III) 植物毒性。 在添加不同濃度 Cr(VI) 之有機質含量較高之內埔系土壤中，Dowex M4195 樹脂之鉻抽出量皆低於儀器之偵測極限，且 Chelex 100 樹脂之鉻抽出量很低，經 XANES 圖譜證實，內埔系土壤中之Cr(VI) 已全部被還原成 Cr(III)，且由小麥之生長並未受到影響，顯示內埔系土壤中由 Cr(VI) 還原而來的 Cr(III) 之有效性並不高。在添加不同濃度 Cr(VI) 且鐵鋁氧化物含量較高之平鎮系土壤中，Dowex M4195 及 Chelex 100 樹脂之鉻抽出量皆隨 Cr(VI) 添加濃度之增加而增加，但Chelex 100 樹脂之鉻抽出量亦比預期中低。由其 XANES 圖譜發現，加入平鎮系土壤之 Cr(VI) 僅部分被還原成 Cr(III)，故小麥所受到之毒害可能同時由兩種形態鉻造成，但由於 Dowex M4195 樹脂之抽出量遠高於 Chelex 100 樹脂之抽出量，因此，造成植物毒害之鉻形態可能仍以 Cr(VI) 為主。建議在含較高有機質之 Cr(VI) 污染酸性土壤中，進入土壤之 Cr(VI) 及其還原後之 Cr(III) 對作物可能造成之影響很小，甚至不會對作物造成危害。但在較低有機質含量的酸性土壤中，進入土壤中之 Cr(VI) 及其還原後之 Cr(III) 皆可能對作物造成毒害，但還原後的 Cr(III) 因為土壤 pH 的升高而致使其溶解度降低，而降低其對作物之危害。 另外，在調整 pH 值並添加不同濃度 Cr(VI) 之供試土壤中，Chelex 100 樹脂之鉻抽出量隨著土壤 pH 值的增加而減少，添加不同濃度 Cr(VI) 之不同 pH 值內埔系土壤之 Dowex M4195 樹脂鉻抽出量皆低於儀器之偵測極限，而添加相同處理之不同 pH 值平鎮系土壤之 Dowex M4195 樹脂鉻抽出量則會隨著土壤 pH 值的增加而增加。內埔系土壤之 XANES 圖譜顯示，不同 pH 值土壤中所添加之 Cr(VI) 皆已被還原成Cr(III)，而平鎮系土壤中之 Cr(VI) 之還原程度則隨著土壤 pH 值的增加而降低，且兩種供試土壤之小麥生長試驗結果與兩種樹脂抽出結果及 XANES 圖譜相符。故建議當土壤中的有機質含量相對較高時（土壤 pH 值介於4 - 6左右），Cr(VI) 之還原主要受土壤中有機質影響，土壤 pH 對Cr(VI) 還原影響並不大，即使土壤 pH 值已經調整至6.4，Cr(VI) 仍可被有機質還原成 Cr(III)（如內埔系土壤）。但若土壤之有機質含量較低，則Cr(VI) 還原程度主要仍受土壤 pH 之影響，其土壤 pH 值越高，則遭受 Cr(VI) 污染後其未還原之 Cr(VI) 含量越高，則其植物毒性亦越強（如平鎮系土壤）。

Cr(III) is less toxic than Cr(VI) and is considered to be a more stable chromium species in the environment. However, under an acidic environment, high levels of Cr(III) concentration in soil were also toxic to some plant species. The availability of soil Cr(VI) is usually used for assessing the phytotoxicity of Cr(VI)-contaminated soils. However, Cr(VI) is favorably reduced to Cr(III) under acid conditions and the availability of Cr(III) in acid soils could be high, thus the phytotoxicity resulting from Cr(III) should not be ignored. Therefore, evaluating the phytotoxicity of Cr(III) and determining the phytotoxicity of both Cr(VI) and Cr(III) in acid soils are essential for assessing the phytotoxicity of Cr. A pot experiment of wheat seedlings was carried out and a chelating exchange resin, Chelex 100, was used to extract Cr(III) from the soils in order to examine the phytotoxicity of Cr(III)-spiked soils. Both acid and alkaline soils were selected in this part of the study, and the soils were spiked with seven levels of Cr(III) (0, 150, 300, 350, 400, 450, 500 mg kg-1soil) for preparing Cr(III)-contaminated soils. Besides, two selective exchange resins, Chelex 100 and DOWEX M4195, were used to extract soil available Cr(III) and Cr(VI) respectively and the X-ray absorption near edge structure spectroscopy (XANES) was used to determine the extent of Cr(VI) reduction to Cr(III). Two acid soils from agricultural areas Pinchen and Neipu were used, the soil pH levels were adjusted to approximately 4, 5, and 6 with addition of CaCO3. The soils were then spiked with six levels of Cr(III) or Cr(VI) (0, 150, 300, 500, 1000, and 1500 mg kg-1soil). A wheat seedling growth experiment was used to assess the phytotoxicity of soil Cr. In the pot experiment, the plant height and the dry weight of the wheat seedlings were adversely affected by the Cr(III)-spiked acidic soils, whereas growth inhibition of wheat seedlings was not found in alkaline soils with the same amounts of Cr(III) added. The amount of soil resin extractable Cr(III) increased with the increase of Cr(III) in the Cr(III)-spiked acidic soils. However, there was no detectable Cr(III) (≤ 0.05 mg kg-1) extracted from the alkaline soils. In addition, the reduction of plant height and dry weight of wheat seedlings, and the increase of plant Cr concentration and Cr uptake showed a pronounced correlation to the amount of soil resin-extractable Cr(III) byChelex 100 resin. The correlation between the relative plant height of the wheat seedlings and the amount of soil resin-extractable Cr(III) in a logarithmic scale was well fitted to an exponential type of dose-response relation model (r2 = 0.97). Therefore, the resin extraction method using Chelex 100 could be a reliable method for assessing the phytotoxicity of Cr(III) in Cr-contaminated soils. The results showed that in Cr(VI)-spiked Neipu soil, both of resin-extractable Cr(III) and Cr(VI) were not detectable and no growth inhibition of wheat seedlings were found. The XANES spectra showed that Cr(VI) added into Neipu soil was almost reduced to Cr(III). It indicates that the availability of Cr(III) formed from the reduction of spiking Cr(VI) in the Neipu soils, which contained higher amounts of organic matter, was low and thus no phytotoxicity effect was observed. On the contrary, in the Cr(VI)-spiked Pinchen soils, only a portion of added Cr(VI) was reduced to Cr(III). The amounts of resin-extractable Cr(III) and Cr(VI) increased with the levels of spiking Cr(VI). The decrease of dry weight and plant height of wheat seedlings was found while the Cr(VI) addition was above 500 mg kg-1soil. The inhibition of growth of wheat seedlings was thought to be related to the increase of amounts of both soil resin-extractable Cr(VI) and Cr(III). Furthermore, the large amount of DOWEX M4195 resin extractable Cr increased with increasing soil pH in Pinchen soil, but the DOWEX M4195 resin extractable Cr was not detected even Neipu soil pH was 6.4., and these results were conformed to the XANES spectra. The results suggest that in acid soils such as Pinchen, which did not have higher amounts of organic matter, the large amounts of both soil available Cr(III) and Cr(VI) resulting from high level of Cr(VI) contamination might be phytotoxic to plant growth. However, in high organic matter content soil such as Neipu soil, even the soil pH is high, Cr(VI) in soil still can be reduced by soil organic matter.